How can I fix my Bosch HBN35GB Cooker (HBN3550GB/02)?

My oven is running painfully slow to cook anything! The cooker does not reach the required temperature when set at say, 220. I need to move the dial to maximum heat and cooking items that usually take 20-30 mins take well over an hour - often 1hr 30 mins. The grill still works but does this double as the main heater or is there another part that most likely needs replacing? Thanks,